Thorium-free, lanthanum borate optical glass

ABSTRACT

Thorium-free optical glass having increased resistance to acids and improved devitrification characteristics, and containing by weight as essential ingredients from 37 to 45% B2O3, from 45 to 53% La2O3, and from 0.5 to 4.5% A12O3.

United States Patent Inventors Appl. No.

Filed Patented Assignee Priority Richard John Parry Southport;

Reginald Dunning, Parbold; Albert Forber, Upholland; George Albert Higharu, Wigan, all of England June 27, 1968 Oct. 26, 1971 Pilkington Brothers Limited Liverpool, England July 26, 1967 Great Britain THORlUM-F REE, LANTHANUM BORATE OPTICAL GLASS 3 Claims, N0 Drawings US. Cl 106/47 Q, 106/54 Int. Cl C03c 3/00 Field of Search 106/47, 54, 47 Q, 47 R Primary Examiner-Tobias E. Levow Assistant Examiner-W. R. Satterfield Attorney-Morrison, Kennedy and Campbell ABSTRACT: Thorium-free optical glass having increased resistance to acids and improved devitrification characteristics, and containing by weight as essential ingredients from 37 to 45% B 0 from 45 to 53% up, and from 0.5 to 4.5% A1 0 THORIUM-FREE, LANTHANUM BORATE OPTICAL GLASS BACKGROUND OF THE INVENTION The present invention relates to thorium-free optical glass.

SUMMARY According to the present invention, there is provided a thorium-free optical glass containing by weight, as essential ingredients, from 37to 45% of B from 45to 53% of 1.3 0,, and from 0.5 to 4.5% of A1 0 and, as optional ingredients, up to 6% of SiO up to 3% ofCaO, and up to 10% ofSrO.

Preferably, the proportion by weight of B 0 is between 38 and 43%. The glass preferably contains fluorine in a proportion by weight of up to 1.5%. The glass also preferably contains SiO in a proportion by weight of at least 2.0%.

The optical glasses of the present invention have increased resistance to acids, and improved devitrification characteristics, as compared with known optical glasses of similar optical properties but which contain thorium.

The thorium-free optical glasses of the present invention may have a refractive index (n,,) between 1.69 and 1.71, and a relative dispersion (V between 55.25 and 56.5. However most of these glasses have n values between 1.691 and 1.705,

' and V,, values between 55,45 and 56.3,

DESCRIPTION OF THE PREFERRED EMBODIMENTS Some examples of glass compositions according to the invention are given in the table below.

TABLE I Percentage by weight of oxides A1203 as A1 0 as SiOz AKOH); AIR; 0110 SrO Samples of these melts were subjected to normal annealing by holding the glass in the annealing furnace at the annealing temperature for about 1 hour, and then switching off the furnace and allowing this and the glass to cool naturally. It was found that the glasses of melts 1 to 14 so produced all had a refractive index (n) value between 1.69 and 55.83. Glasses of melts l5 and 16 had slightly lower n, values and slightly higher V values. For example, melt 16 has a n value of 1.69143 and a V value of56.26.

Samples of these same melts were also subjected to fine annealing by cooling at the rate of 20 C. per day from the annealing temperature, and higher values of n up to 1,709, and of V, up to 56.5 were obtained with glasses fine annealed in this way. For example, melt 15, when fine annealed in this way has a n, value of 1,69823 and a V, value of 56.2.

It has been found that the inclusion of up to 1.5 percent fluorine, e.g. by the use of .AlF to provide some or all of the A1 0 assists in melting any glass which contains silicon and also tends to give higher n, and V values than the fluorinefree glass of otherwise similar composition.

The effect of fluorine was checked in the case of melts Nos. 9 to 12-. As will be apparent from table 1, all these melts contain identical amounts of B 0 La O SiO A1 0 and SrO, and only differ in the increasing proportions of fluorine introduced as All-" Thus while in melt No. 9 all the A1 0 is provided for by A1(OH) in melts Nos., 10 and 11 the A1 0 is partially provided for by A11 and in melt No. 12 all the A1 0 is provided as Alf Samples of glass from melts 9 to 12 annealed in identical conditions, and the n,, and V, values were then carefully checked, giving the following results:

TABLE II No. of

Melts n, V,

B,O, from 38.73% to 42.89%, La o, from 46.92% to 52.83%, A1,O from 0.75% to 4.35%,

and at least one oxide of the group consisting of 2 to 6% SiO,, 1.08 to 2.43% CaO, and 1 to 7.98% SrO.

2. An optical glass according to claim 1, including fluorine in a proportion by weight of up to 1.5 percent.

3. An optical glass according to claim 1, having a refractive index (n between 1.691 and 1.705, and having a relative dispersion (V between 55.45 and 56.3.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,615,762 Dated October 26 1971 Inventor) Rlchard J. Parry et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

IN THE SPECIFICATION Column 1, Line 54 Insert after 1.69 and 1.705 and a relative dispersion (V value between 55.45--

Column 2, Line 22 Insert after 12 were fine-- Signed and sealed this 6th day of June 1972.

(SEAL) Attest:

EDWARD M.FLETC.=IER, JR. ROBERT GOTISCHALK Atteating Officer Commissioner of Patents USCOMM-DC 50375-969 RM PO-IOSO (10-69) a u s sovzmmzm- PRINTNG OFFICE-1969 o-sss-su 

2. An optical glass according to claim 1, including fluorine in a proportion by weight of up to 1.5 percent.
 3. An optical glass according to claim 1, having a refractive index (ne)between 1.691 and 1.705, and having a relative dispersion (Ve) between 55.45 and 56.3. 